Solvation enthalpy phase transition

In a DSC scan, the difference of energy input (heat flow) into a sample and into a reference material is plotted as a function of temperature. In a plot of heat flow vs. temperature, endothermic minima correspond to desolvation of solvates or phase transitions of polymorphs, and exothermic maxima correspond to crystallization or decomposition. Integration over the area of a transition feature yields the associated transition enthalpy. 

This section is concludedby discussing another type of phase transition, known as solvation, which refers to the dissolution of a gaseous substance in a liquid solvent. We already met this concept when we were deriving the enthalpy of reaction 2.1 (see figure 2.1 and sections 2.4 and 2.5) the solution enthalpy of gaseous O2 in water, AS]n7/(2), can also be called the solvation enthalpy of O2 in water. Note that when the solvent is water, the word solvation is often replaced by hydration. 

ELDAR contains data for more than 2000 electrolytes in more than 750 different solvents with a total of 56,000 chemical systems, 15,000 hterature references, 45,730 data tables, and 595,000 data points. ELDAR contains data on physical properties such as densities, dielectric coefficients, thermal expansion, compressibihty, p-V-T data, state diagrams and critical data. The thermodynamic properties include solvation and dilution heats, phase transition values (enthalpies, entropies and Gibbs free energies), phase equilibrium data, solubilities, vapor pressures, solvation data, standard and reference values, activities and activity coefficients, excess values, osmotic coefficients, specific heats, partial molar values and apparent partial molar values. Transport properties such as electrical conductivities, transference numbers, single ion conductivities, viscosities, thermal conductivities, and diffusion coefficients are also included. 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

Fig. 13. Reaction profile for DMSO exchange on solvated Li+ enthalpy B3LYP(CPCM)/6-311+G including thermodynamic contributions at B3LYP/LANL2DZp gas-phase energy values B3LYP/6-311+G including AZPE at B3LYP/LANL2DZp, in parentheses T.S. = transition state (92).

The rate enhancement by anionic micelles is the consequence of a decrease in both the enthalpy and the entropy of activation (Graham and Leffler, 1959), and thus the catalysis is understandable in terms of a greater rate of racemization in the micellar phase than in the bulk solvent resulting from decreased solvation requirements and less destabilization of the transition state in the micellar system relative to that in water. 

A detailed molecular-level understanding of the role of solvation on the nature of Sn2 reaction pathways has been revealed only during the last decades. Fig. 5 compares the gas-phase Sn2 enthalpy diagram with two minima, first proposed by Brauman et al. [474], with the more familiar single transition-state diagram obtained in solution. 

We have recently reported a detailed discussion on solvation effects in this particular reaction [17]. Briefly, the experimental activation energy value at 298 K in the gas phase has been reported to be 19.7 kcal/mol [70]. In toluene, the experimental activation enthalpy was reported at 15.8 1.4 kcal/mol, with an activation entropy of —38 4 cal/mol K [71]. Four possible reaction pathways are possible for the acrolein and s-cis butadiene reaction. Consistent with previous conventions [17,72,73], the transition structures are denoted as NC (endo, s-cis acrolein), XC (exo, s-cis acrolein), NT (endo, s-trans acrolein), and XT (exo, s-trans acrolein), as illustrated for the parent reaction in vacuum in Fig. 5. 

The differences in the mechanisms in the gas and solution phases necessarily arise from differences in solvation. In the gas phase the nucleophile and electrophile solvate one another prior to reaction, and achieving the correct orientation between the nucleophile and electrophile for backside attack leads to a substantial entropy barrier. The rate constants therefore have little correlation with the enthalpy of the reaction. In solution, however, the electrophile and nucleophile are solvated prior to the reaction, the transition state is solvated during the reaction, and the product is solvated after the reaction. This yields rate constants that correlate with the heat of the reaction. 

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